DESCRIPTION:(provided by applicant) This proposal uses the marine mollusc Clione limacina as a model with which to study a common behavior; orientation to the gravitational field. Clione orients itself in the water column, in the heads-up position by means of wing and tail movements. These movements respond to signals from the statocyst organ to maintain a vertical position. The statocyst organ contains a small stone, the statolith, that rests on sensory hair cells and is moved around on these cells by changes in the animal's position in the gravitational field. After a disturbance which changes body position, signals from the statocysts are integrated in the cerebral ganglion which generates the correct spatio-temporal pattern of impulses to the wing and tail motor neurons thus reducing the statocyst signal to its previous condition. Although a seemingly straight forward negative feedback loop appears to be the basic mechanism involved, the behavior actually requires a complex coordination between three functional elements; equilibrium receptors, CNS interneurons and the wing and tail motor neurons. In addition to the equilibrium response, the animal can engage in another related behavior known as hunting. During hunting, the normal equilibrium response disappears and the animal engages in what appears to be random circular sweeps of its environment. We hypothesize that the basically unstable vertical orientation mechanism requires coordination between the CPGs for both wing movements and tail movements and the hunting mechanism involves the activation of efferent connections between the cerebral ganglion and the lattice of receptor cells in the statocyst. To prove this hypothesis we will use a combination of behavioral, electrophysiological and modeling studies. We also hypothesize that specific feedback among cerebral ganglia and statocyst receptors together with inhibitory interconnections among receptor neurons is able to stimulate a complex hunting search behavior that resembles the chaotic motion of a pendulum in three dimensional space. The behavioral analysis will quantify the phase relationships between wing and tail movements during stable and perturbed behaviors. The electrophysiology is necessary to describe the cellular responses to statocyst output and the computational analysis will employ three different models to determine which best captures the salient features of the equilibrium and hunting responses most accurately and to provide a theoretical basis for the production of the two different behaviors.